1. Field of the Invention
The present invention relates to a hydraulic control system of an automatic transmission, in particular, to a control for detecting an inertia phase of the shift operation to gradually reduce a hydraulic pressure for frictional elements actuated in the shift operation to thereby suppress a shift shock.
2. Description of the Prior Art
Generally, in an automatic transmission for an automotive vehicle, an engine output is introduced through a torque converter to a shift gear mechanism in which a plurality of frictional elements are selectively switched to be actuated to change a power transmitting path thereby establishing a specific shift stage automatically in accordance with a vehicle operating condition, such as vehicle speed, acceleration stroke and the like. As disclosed in Japanese Patent Public Disclosure No. 2-76968, laid open to the public in 1990, the frictional elements are engaged and disengaged by means of a hydraulic control pressure produced in a hydraulic control circuit which is provided with various control valves. Introduction of the hydraulic control pressure is controlled in accordance with a shift pattern provided based on the vehicle operating condition.
The hydraulic control pressure is changed to have an leveled pressure condition in which an engaging hydraulic pressure to an ongoing frictional element is gradually changed. The leveled pressure condition during the shift operation is produced by virtue of an accumulator which is disposed in a hydraulic line through which the engaging pressure is introduced to the frictional element to be engaged. Thus, the control pressure is gradually changed in the leveled pressure condition so that a shift shock accompanying the engaging action of the frictional element can be effectively eased. Alternatively, in order to obviate such shift shock, it has also been known that a line pressure in the hydraulic control circuit is reduced in accordance with an engine load or a kind of the shift operations, such as a 1-2 shift operation in which the shift operation is made from the first to the second shift stage, a 2-3 shift operation from the second to third shift stage, 3-4 shift operation from the third to fourth shift stage and the like.
The conventional hydraulic control system of the automatic transmission is however disadvantageous in that a gradient of the control pressure during the leveled pressure condition takes a substantially constant value which is not variable because the leveled pressure condition is produced by a resilient force of a spring provided in the accumulator irrespective of the control of a decrement of the line pressure of the hydraulic control circuit. Meanwhile, a shift operation is generally done from one shift stage to another in such a manner that one frictional element engaged in the one shift stage is controlled to be disengaged and another frictional element disengaged in the one shift stage is controlled to be engaged so as to establish the other shift stage. Therefore, when the one frictional element is actuated to be disengaged reducing a transmitting torque amount therethrough in the case where the other frictional element is actuated to be engaged increasing the transmitting torque therethrough but yet produced enough to make up the reduction of the transmitting torque amount due to the disengaging action of the one frictional element, the turbine speed or input speed of the transmission is increased. Then, as the transmitting torque or engaging force of the other frictional element is increased, the turbine speed starts to be reduced. When the turbine speed is substantially increased after the shift operation starts, it is referred to as a torque phase of the shift operation. When the turbine speed is reduced following the torque phase, it is referred to as an inertia phase. Thus, it will be understood that the engaging action of the other frictional element starts when the phase is changed from the torque phase to the inertia phase. It will be also understood that the engaging action is dominated by the gradient of the control pressure during the leveled pressure condition which is fixed by the resilient force of the spring of the accumulator irrespective of the control of the line pressure as aforementioned. As a result, a shift shock is inevitably produced.
In Japanese Patent Public Disclosure No. 3-199762, laid open to the public in 1991, a line pressure is controlled to eliminate an abrupt hydraulic pressure change as changed to have a peak portion appearing in an initial and a final stages of the leveled pressure condition. However, This cannot change the gradient of the hydraulic pressure change. Therefore, this control is not able to eliminate an abrupt increase of the transmitting torque amount, that is, he engaging force during the leveled pressure condition so that a shift shock is still produced.